Then start MyUnity and switch to themes, under icons select "Faenza-Ambience" if
you use Ambience as theme. Otherwise the icons in the panel don't match, which means they are black on dark background. If you prefer the Radience theme, choose "Faenza" or "Faenza-Radience". Changes should become visible immediately:

If you don't want to add a PPA as described above you can download the zip file on its homepage.